The present invention relates to systems and methods for surging fluids downhole in an earth borehole, and for tools especially adapted for use in such systems and methods.
Surging is a technique useful in completing, treating and testing oil and gas wells. For example, backsurging is used to clean perforations by producing a high fluid pressure differential at the location of the perforations which results in turbulent flow through the perforations into the well. The technique is also useful for initiating flow tests which serve to estimate oil and gas reservoir extent and measure flow rates for a given formation. In sand control operations, the technique is used for forming a void outside the well casing so that gravel can be forced into the void to form a sand filter.
A number of different valves for use in surging wells have been described. For example, various types of ball valves, check valves and flapper valves have been proposed for use in surging techniques. Because a large pressure differential is present across the valve prior to actuation, valves of this type are prone to leak. For this reason, it has been proposed to use valves employing a frangible member which is shattered when the valve is opened. Such valves are better able to withstand high pressure differentials without leaking. However, loose pieces are formed by shattering the disc and these can clog or plug off the pipe string which is used to run the surging tools into the well.
In one such valve described in the prior art a frangible disc is broken by a cutter forced against the disc by a piston. The piston is powered by a fluid pressure differential across the piston produced by elevating upper annulus fluid pressure over fluid pressure trapped beneath the valve. The piston is free to move as the pressure is thus increased in the upper annulus. Accordingly, the cutter is urged against the disc as upper annulus fluid pressure is increased, so that when a sufficient pressure differential is produced, the cutter breaks through the disc, thus shattering it and releasing pieces of the disc into the flow of fluid through the valve.
There are several disadvantages in this design. Pieces from the shattered disc can form a blockage of the pipe string interfering with operations. Where a high pressure is trapped beneath the valve, it is necessary to produce a relatively high fluid pressure differential to break the disc. This may not be feasible if the necessary pressure level exceeds the pressure level which the casing can safely withstand. In order to accommodate such situations, breakable discs of differing thicknesses have been provided. Accordingly, a relatively thin disc will be used where it is not possible to safely produce a high pressure differential for actuating the tool. However, the availability of discs of varying thickness creates the possibility that a disc of the wrong thickness may be used. The result may be the spontaneous rupture of the disc if it is not sufficiently strong to withstand hydrostatic pressure in the well. If a disc having too great a thickness is used, it may not be possible to break it with the application of safe pressure levels in the upper annulus.
Where it is desired to surge into a chamber of limited volume, the prior art utilizes a second surge valve incorporating a second breakable disc forming the upper extremity of the surge chamber. In contrast to the operation of the lower valve, the upper valve is operated by increasing tubing pressure above the valve so that the tubing pressure sufficiently exceeds upper annulus pressure to force a cutter through the breakable disc. It will be readily appreciated that this valve shares many of the same disadvantages and limitations of the previously described valve. In addition, the use of tubing pressure to actuate the upper valve can force fluid and debris back into the perforations thus damaging the formation. It is also possible that the well's mud system can become contaminated by hydrocarbons if the packer is unseated before opening the upper valve to avoid forcing fluid and debris back into the perforations.
In a different prior art surging system, a surge chamber is formed between two removable plugs. Applying pressure to the annulus unseats the bottom plug in order to open the surge chamber to formation fluid. Thereafter, tubing pressure is increased to unseat the top plug so that both plugs are forced down the tool string and out the bottom of the tubing. Aside from forcing debris and fluid back into the perforations and the formation, this technique is not well adapted for producing a large pressure differential across the perforations, since fluid surges around the bottom plug as it is unseated.
In a further prior art surging system, a lower surge valve has a breakable disc shattered by dropping tubing weight on a disc cutter to force it through the disc. An upper surge valve has a breakable disc opened by dropping a bar down the tubing to strike the disc and shatter it. Such systems are expensive. Also, well operators prefer to avoid manipulating the tubing string. Where scale and other debris from the tubing string settle on the disc of the upper valve, the bar may not be able to shatter it upon impact.